Means for and method of reducing interference



Jan. 26 -1926. 1,570,770

H. NYQUI ST MEANS FOR AND METHOD OF REDUCING INTERFERENCE Filed March 21922 J z LI INVEN TOR.

WTTORNEY Patented den. 25, 1923.

' nanny nreuisr, or min-runner. NEW YORK, ASSIGNQR 'ro AMERICANTELEI'HONE AND TELEGRAPH comranr. A CORPORATION OF NEW YORK.

m ssus .ron AND Marxian or REDUCING mcrnnrnnnucnf Application filedMarch ,20, 1922. Serial No. 545,295.

To all "iv/mm 2T1 rum concern Be it known that I; HARRY NYQUIST,residing at Elmhurst. in the county of Queens and State of New York,have invented cer- 5 tain Improvements in Means for and .Methods ofReducing Interference, of which the following is a specification.

This-invention relates to multiplex signaling, and more particularly tomeans for and method of reducing interference between the channels of amultiplex signaling system.

Heretofore, in the operation of multiplex carrier systems, it-has beenfound that the repeater tubes employed at points along the line fortransmitting the carrier frequencies, where the same tube is used totransmit the frequency of a number of channels, tended toimpose a limiton the number of channels that might be employed over a single circuit.This is for the reason that the tubes do not I have a straight linecharacteristic and a zcertain amount of distortion occurs, whichproduces second harmonics of the carrier frequencies and also sum anddifference frequencies. difference frequencies, in many instances,assume such amplitudes as to cause interference in the channels whosefrequencies are near those of the interfering harmonics, etc;

Interference of this character"becomes especially serious in a carriertelegraph sys tem involving a large number of channels employing carrierfrequencies within the 35 usual voice range, for in such a case it isalmost, essential .to economic working of the system that the channelsbe narrow and closely spaced, and that all the channels he amplified bythe same repeater tube. Even where the circuit is to be used forordinary voice transmission, the distortion, while not producing resultsas serious as in the case just. n'ientioned. may besufficientlyundesirable to require correction, especially where thecharacteristics of the repeaters depart. materially from the desiredlinear characteristie. In accordance with the present invention, it isproposed toovercou'i-c this diificulty by pollin'g successive repeatersoppositely, so that the currents under consideration will besubstantially neutralized in two successive repeaters. This result maybe readily accomplished by reversing the connections between the tube toone repeater and the tube These harmonics and sum and to the nextsucceeding repeater. In this case, the harmonics and sum and differencefrequencies generated by the tube of the second repeater will besubstantially 180 degrees out of phase with the harmonics generated inthe first repeater" and will, therefore, tend to neutralize them. Thiswill not be true for third harmonies or any distortion arising from thepresence of a third power term in the repeater characteristic, but ingeneral, components resulting from the higher power terms of therepeater characteristic are of such small magnitude that they may beneglected.

The invention may now be more fully understood by reference to thefollowing description, when read in connection with the accon'q'ianyingdrawing, Figure 1 of which illustrates the arrangement of the terminalequipment of a multiplex carrier telegraph system operating within thevoice range, and Fig. 2 of which illustrates the arrangement of twosuccessive repeaters in a line-associated with terminal equipment, suchas that of Fig. 1. i 4

Before discussing the theory underlyin the present invention, a briefdescription 0 the circuit. arrangements to which the inventionappertains and of the simple change in circuit arrangement for carryingout the invention will begiven.

Referring to Fig. 1, L and Lt, designate two transmission linesoperating on a four-'- wire basis, so that one line transmits currentsin one direction and the other line transmits currents in the otherdirection At the. terminal station each of these lines includesa one-wayterminal repeater, as indicated at R and R and these repeaters may be ofany well-known type, such, for example, as vacuum tube repeaters. Thedetails of the circuit arrangement of the repeaters may be as shown,forexampla in Fig. 2, which will be described later. The inputside oftherepeater R, is associated with bus connections '10 from whichbranches lead through various channels, such asthe transmitting channelTL. In a similar manner, the output side of the repeater R, isconnected'to bus-bars 11 'from which branch connections extend to thevarious channels such as thereceiving channel RL..

, said sections including series resonant elements and shuntanti-resonant elements.

In order to give the filter as great an impedance as possible tofrequencies outside of the range which it is desired to transmit, thesection of. the filter adjacent the multiple connection to the bus-barsis made a full series section, as indicated. Filters constructed inthis'manner will have a fairly sharp cut-0d and the attenuation will bevery eat for frequencies on either side of the limits of the band offree transmission,

Thejransmitting channel TL is associated through atransformer 12 with asource of carrier frequency S and the transformer 12 is connected withthe'channel TL in such a v manner that the frequency supplied to thechannel from the source S may be interruptc'd'by means of a contact 13of a polar transmitting relay 14, whose windings are arranged one in"the telegraph line L and the other in the connection to the balancingnet'- work N. The receiving channel EL includes a detector D, which maybe of any well-known type, such for example as a vacuum tube detector,and the output G11- cuit of the detector includes a receiving relay RRwhose contact is arranged to relay the received impulses to themidpoints of the windings ofthe polar relay 14 for transmission over thetele aph line L without operating the transmitting relay.

. By means of this arrangement, telegraph signals incoming from the lineL will operate the'polar relay 14 to interrupt the con nection of thesource S to the channel TL, so that a'carrier frequency interrupted inaccordance with the telegraph signals will be t'ransntiitted through thefilter TF to the bus-bars l0 and thence throughthe terminal repeater RVVto the line l'l Similar carrier frequencies transmitted in the oppositedirection from andis'tant station will be received from thelin e la, andamplified by the terminal repeater RE bcfore being impressed upon the[bus-bars 11. The fret uru relays the Morse signalstothc receivinglinerltlj Without interfering with the. terwhere i is the current in thepla.

. sesame line sections 20, 21 and- 22. Each repeater W as showncomprises two tubes'in tandem and a potentiometer associated with theinput circuit of the first tube. A transformer associates the input ofthe first tube with the potentiometer, and another transformer isinterposed between the potentiometer and the line section. The output ofthe second tube of each repeater is similarly connected with the nexttransmission line section by a transformer. While the circuitarrangezucnt of Fig. 2 has been described as being used in connectionwith the multiplex telegraph system of Fig. 1, it is obvious that thecircuit'may be employed for ordinary voice transmission. In eithercasethe distortion produced by harmonics and sum and differencefrequencies of the various fre quencies transmitted through therepeatersmay be overcome by properly paling the repeaters in the mannerabout to be described.-

second harmonics and sum and difference frequencies of successiverepeaters are caused to oppose each other, may be accomplished insei'eral different ways. The line wires of the line section interveningbetween two repeaters might be transposed at any desired point, or theconnections of some transformer or other piece of apparatus to thecircuit might be reversed. Between the repeater R, and repeater R threetransformers 23, 24 and 25 are interposed. and the any one of thesetransformers mig t be reversed in order to obtain the desired result. Asillustrated in Fig. 2, transformers 23 and 24 are poled in the-same way,but the connections from the second Winding of the transformer 25 of thegrid of the first tube of the repeater R, are transposed, therebyreversing the poling of the transformer This will result in'producingsecond power effects in the repeater R, which are opposite in phase tothe second power effects produced in the repeater R The reason why thesimple reversal of connections above described will tend to.

cling ofelimin'ate interference duetof secondharwill be clearfrouithefollowingtheoretical discussion of=the principles involved.

Suppose we are 'not consideringdistortion arising from terms higherthan" the second power. Thenthe repeater istic may be represented by vand e is the alternating voltage'o'n the rid.

Suppose further that there is no press minalrelay, as. has ,alreadynbecndescribed. shiftrin,.cither the line or the repc'atcr,--that is, noshifting of the phase with respect to Referring now to Fig. 2, a line isdisclosed v which may be considered as typical of lines R, areillustrated at successive repeater stasuch as L, and L Two repeaters R,and

tions and serve to interconnect-successive vocal transmission) at allparts of the we character- 3 I I .120

cuit have the same relative phase with which they started out. Now letus consider the. action of a Wave made up of two frequencies of equalamplitude. Such a wave may be expressed i 6 (cos gate-00551515) (2)where p, and go, represent Q'rr times the fre quencies of the individualcarrier waves (or two component volce frequencies). Substituting fromequatiqu in equation (1),.

we have In equation (3) the constant a represents a direct current terma hich is not transmitted through the transformer in the output of therepeater from the line section to the next repeater. The expression 6E(cos p ta-cos Ps 0E (cos p t+oos 1 20 r g a: ill (cos pi P2051 ment(which is only approximate) it should be remembered that in equation-(4) the expressmn I 533 (cos p H-cos p 6) is the term representing thean'iplified com- )onent corresmnding to the ori inal wave and the onlyterm with which We "are concorned in amplification. We had a perfectamplifier, that is. an amplifier having a straight line Qhm'acteristic,the second Y power term 0E (cos p rf cos p 2?) algebraic expression (4).Conseqr'rcutly. the

voltage applied to the grid of the succeeding amplifier may be expressedmbE (cos p z+cos 1 'mcE (cos p t-rcos 12 i)" (5) where m a factorrepresenting the relation between the voltage applied to the grid of thesucceeding amplifier and the current flowing in the output circuit ofthe prccediug' amplifier. lt' we analyze the expression itwill be seenthat the first term ,thereof is of the same form as the voltageimpressed upon the grid of the pryccding' amplifier as given by e nation(2). the only difference being that tie term in equation (5) ismultiplied by the factor 7117/. If this factor be made equal to unitythe lil'sl term of equation (5), (which represents the voltage appliedto the grid of the succeed ing tube) will be identical with the voltageapplied to the grid of the preceding tube. The factor Z) is atermdepending upon the amplification of the preceding tube, but the factor mdepends upon the transmission charactoristics of'the line and is alsocontrolled by the adjustn'icnt of the potentiometer in the input circuitof the grid of the succeeding amplifier. By proper adjustment of thepotentiometer the factor mb ma be made unity, and we then have as thealternating voltage on' the grid in. the second repeater the expression2 (cos p t+cos gm) (6) andother higher'power terms would not be presentat all. Evenfin the practical case now under consideration, where aslight amount of distortion docs actually occur, the second power tern'iis quite small as compared with the first power term. Consequently, itis quite justifiable to sir in connection with equation (6) that, sincethe first power term of the right-hand side of said equation isidentical with the original wave of equation (2), the voltage impressedupon the grid of the succeeding tube. is practically equal to thevoltage impressed upon the grid of the preceding tube. Therefore, thegain of the repeater is substantially equal. to the loss in the line. v

The characteristic of the second repeater may also berepresentcd by anequation similar to equation (1). Hence we have, by substituting thevalues given by equation (6) in the equation corresponding to equation(1) for the second repeater,

inc: BE (cos p t+ cos 9 5);: 0E (cos cos 10 .2 I c[E (cos p 6 cos p t) fp t+cos jp t) 1 (7) l serum/o comes E (cos pJ-l-cos 156 l l terms in 'Eand E". e 3) Hence the space current takes the form i=c+bE (cos p t+cospull-2 0E (cos p,t+ cos umterms in E and E (9) where there. is an evennumber of reversals. Where there is an odd number of reversals, thespace current takes the form terms 111E and E (10) gb'lpifl'lflgequations (9) and (10), it will: bgsecn that the term 7 E (cos p t+eosput) cies will be suppressed. That the expression 1 (cos p t +005 p n 3is responsible for the second harmonics and sum and differencefrequencies will be up parent from a simple algebraic transformation, hwhich this expression reduces to the fol owing:

Analyzing the four terms on the right-hand side of the equation (11), itwill be seen that the first and last of these four terms representsecond harmonics of the frequency 12 and 32 while the second and thirdterms represent the frequencies corresponding to the sum and differenceof these two frequencies, respectively. Q s

It will be obvious that the generahprinciples herein disclosed may beembodied 1n many other organizations widely d fl'erent from thoseillustrated, without departm from the spirit of the invention as defineinthe following claims.

What is claimed is:

-1. In a'multiplex system in which 51 plu mlity of carrier frequenciesare simultaneously transmitted throu h succcssivcgepcaters'in atransmission line, theme ml of eliminating secondlmrmouics and sum anddiiference frequencies produced by the dis,- tortion of the repeaters,which consists in impressing the several carrier frequencies onsuccessive repeaters in opposite phase relution. v

2. in a multiplex system in which u plurality of carrier i'requeucigsare transmitted through successive repeaters in a transmission line, the.nie'thod of eliminating second harmonics, which} consists inlIITPIOSSlHg the severstl carrier frequencies on successive repeaters inopposite ,phase relation.

3. In a, multiplex "system in whidlf'a plurality of carrier ire uenciesaresimultm neously transmitted. t iroughsuccessive repeaters inntransmission line, the method of clinfinuting sum and differencefrequencies, which consists in impressii'ig the several carrierfrequencies on successive repeaters in opposite phase relation.

- rlln s. multiplex system in which a plurality of carrier frequenciesare transmitted through successive re esters in s transmission line, themetho of eliminetin even power components due to the distortion of therepeaters which consists in reversing the. connections het weensuccessive repeaters 5. In a multiplex carrier system, a transmissionline, means for transmitting e plurality of carrier frequencies overseidtrens mission line, a plurality of repeaters included in saidtransmission line, the connections of the successive re eaters being soarranged with respect to one other that the carrier frequencies will beimpressed upon each succeeding repeater in'opposite. phase relation to,that in which they were impressed upon the preceding repeater of theseries. a ,5;

' 6. In omultiplex carrier system, a transmission line, means fortronsmitting a pinrality of carrier frequencies over said trouss miss online, a plurality of repeaters included in said transmission line, theconnections of each successive repeater 'beiug re versed withrespcct tothe repeater imme diately preceding, whereby the currier'frequencieswill be impressed upon each] recessive repeaters in. opposite phaserelatiom 8, In a. signaling system inwhich a range of frequencies istransmitted through successive re esters in a transmission line, themethod 0 eliminating second harmonics and genome sum and differencefrequencies producd by the distortion of the repeaters, which consistsin impressing the "various transmitted frequencies within the range uponsuccessive repeaters in opposite phase relation.

9. In a signaling system in which a range of frequencies is transmittedthrough successive repeaters in a transmission vline, the method ofeliminating frequencies corresponi'ling to the sums and difierences ofcornponent :trequencies Within the transmitted range, which consists inimpressing the frequencies to be transmitted upon successive repeatersin opposite phase relation.

10. In :2. signaling system in which a range of frequencies istransn'iitted through successive repeaters in a transmission line, themethod of eliminating even-power c0mponents, due to the distortion ofthe repeaters, which consists in reversing the connections betweensuccessive repeaters.

1.1.111 a transmission system, e transmission line, meens fortransmitting a bend of frequencies over ssid transmission line, a

plurality of repeaters included in said transmission line,theconnections of the successive repeaters being so arrenged withrespect to each other than the component frequencies of the band will beimpressed upon each succeeding repeater in opposite phase relation tothat in which they were impressed upon the preceding repeater of theseries.

12. In a transmission system, a transmission line, means fortransmitting a bend of frequencies over said transmission line, a,plurality of repeaters included in such transmission line, theconnections of each successive repeater being reversed with respect tothe repeater immediately preceding, where-.

by the component frequencies of thebend will be impressed upon eachrepeater in op posite phase relation to that in which they wereimpressed upon the preceding repeatelx.

In testimony whereof, I have signed my name to this specification thisl8th'day of March, 1922.

Her nroursr.

